Photomask and method of exposure using same

ABSTRACT

A photomask has a plurality of main holes which pass a prescribed light beam that is shone onto positions that make up a plurality of pattern parts, at locations that are opposite a plurality of pattern parts for said semiconductor device, this photomask also having a plurality of minute auxiliary holes, which pass a light beam of a degree that is not transferred at the time of exposure, these auxiliary holes being disposed between the main holes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a photomask used in an apparatus formanufacturing a semiconductor integrated circuit and to a method whichuses this photomask to expose the pattern of a semiconductor integratedcircuit.

More specifically, it relates to a mask for forming the contact holes ofa memory device and to a contact hole exposure method.

2. Description of Related Art

In the past, the increasing level of integration of semiconductorintegrated circuits has resulted in a rapid shrinking of circuitpatterns used therein. However, the lithography technology that uses aprojection-type exposure apparatus has a limit of resolution that isrelated to the wavelength of the light source.

In recent years, there have been various methods proposed for achievingultra-high resolution, such as the method of using a phase-shift mask orusing illumination of a particular shape to improve the resolution.

One known half-tone phase-shift mask for the purpose of improving theresolution for hole patterns, is known by its disclosure in the JapaneseUnexamined Patent Publication (KOKAI) No. 4-136854. With a half-tonephase-shift mask, a translucent phase-shift mask is used in place of theusual light-blocking film.

However, when a half-tone phase-shift mask is used, because of the verysmall amount of light that passes through the translucent phase-shiftmask, there is a region of strong light intensity surrounding the holepattern, this being known as a side lobe.

In the case of a memory device, therefore, in which the spacing betweencontact holes is narrow, as the size of features becomes smaller, theseside lobes overlap with one another, so that unwanted side lobes aretransferred onto the resist.

Another known method of improving the resolution for hole patterns otherthan the use of a half-tone phase-shift mask is that, for example, whichis disclosed in the Japanese Unexamined Patent Publication (KOKAI) No.4-268714.

This method, as shown in FIG. 6, makes use of a mask onto which areprovided auxiliary holes 5, which are below the resolving limit of theprojection lens, in the vicinity of the area surrounding the holepattern 4.

By exposing through this mask using a ring-shaped light source 10 asshown in FIG. 5(A) or a four-point light source 11 as shown in FIG.5(B), it is possible to improve the hole resolution.

These types of masks are effective for use with a hole pattern ofsparsely distributed isolated holes.

Another method is that which is disclosed in the Japanese UnexaminedPatent Publication (KOKAI) No. 62-67514, in which is presented aproposal, as shown in FIG. 7, of a mask which is provided with auxiliaryshifters 9, which surround the hole pattern 4, and which shift the phaseof the light passing therethrough by 180 degrees, these being of a sizethat is smaller than the resolving limit of the projection lens.

This mask imparts a phase shift effect to isolated holes, and improvesthe resolution when used with illumination having a small numericalaperture σ of 0.3 or smaller in an illumination optical system.

A mask with auxiliary holes or auxiliary shifters such as shown in FIG.6 and FIG. 7 is effective with respect to isolated holes.

However, the arrangement of holes in the contact hole pattern of amemory device, such as shown in FIG. 3, has parts in which the holes arearranged in high concentration.

For example, in the case of the capacitive contact hole that is shown inFIG. 3(C), it is not possible to provide auxiliary holes or auxiliaryshifters that surround the hole pattern 4.

Accordingly, it is an object of the present invention to provide aphotomask that alleviates the above-noted problems which accompanied theprior art and which improves the resolution of a contact hole pattern ofa memory device, and to provide an exposure method which uses thisphotomask.

SUMMARY OF THE INVENTION

To achieve the above-noted object, the present invention has thefollowing basic technical constitution.

Specifically, the first aspect of the present invention is a photomaskfor the purpose of forming a plurality of pattern parts on at least partof the pattern of a semiconductor device, this mask having a pluralityof main holes at locations which are opposite the positions at which theabove-noted plurality of pattern parts are to be formed, each one ofthese main holes passes a prescribed light beam that is irradiatedthereonto, this photomask further having at the plurality of minuteauxiliary holes between the main holes and each of which passes a lightbeam therethrough that is of a degree that does not transfer to asurface of a substrate at the time of exposure.

The second aspect of the present invention is a photomask which has anoptical phase shifter provided at at least one of the mutually adjacentmain and auxiliary holes of the plurality of main and plurality ofauxiliary holes.

Additionally, the third aspect of the present invention is a method ofexposure which uses a photomask according to the first aspect of thepresent invention to perform exposure using deformed illumination inwhich the surrounding area of the illumination light source is bright.

The fourth aspect of the present invention is a method of exposure whichuses a photomask according to the second aspect of the present inventionto perform exposure using illumination from a light source with anillumination optical system having a small numerical aperture.

By adopting the above-noted technical constitution, a photomaskaccording to the present invention can be used, for example, as aphotomask having the contact hole pattern for a memory device.

As a specific example of the configuration thereof, matrix lines wouldbe drawn between the above-noted contact hole patterns to connect thoseholes, thereby establishing the points of intersection of the matrixlines with minute auxiliary holes that are not transferred to a surfaceof a substrate when it is exposed being added at some or all of thematrix points at which there is no contact hole pattern.

In a method according to the present invention of performing exposure,using a prescribed light beam, so as to form a prescribed pattern in,for example, a resist, for the purpose of forming a circuit part of asemiconductor, in the case in which a photomask without the above-notedphase-shifting means is used, it is desirable that the exposure beperformed using deformed illumination, in which the peripheral area ofthe illumination is bright and, in the case in which a photomask havingthe above-noted phase shifters is used, a conventional light source isused, but with the light beam considerably contracted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a plan view which shows an example of the configuration ofthe pattern of a semiconductor circuit to be described using a photomaskaccording to the present invention,

FIG. 1(B) is a plan view which shows the configuration of a specificexample of a photomask that is used for the bit line contact holesaccording to the present invention, and

FIG. 1(C) is a plan view which shows a specific example of a photomaskthat is used for capacitive contact holes according to the presentinvention.

FIG. 2(A) is a plan view which shows an example of the configuration ofthe pattern of a semiconductor circuit to be described using a photomaskaccording to the present invention,

FIG. 2(B) is a plan view which shows another specific example of aphotomask that is used for the bit line contact holes according to thepresent invention, and

FIG. 2(C) is a plan view which shows another specific example of aphotomask that is used for capacitive contact holes according to thepresent invention.

FIG. 3A is a plan view which shows an example of the configuration of apattern of a semiconductor circuit to be described using a photomaskaccording to the prior art,

FIG. 3(B) is a plan view which shows the configuration of a specificexample of a photomask that is used for bit line contact holes accordingto the prior art, and

FIG. 3(C) is a plan view which shows the configuration of a specificexample of a photomask that is used for capacitive contact holesaccording to the prior art.

FIG. 4 is a graph which shows the difference between the focal depth offield of a prior art photomask and a photomask according to the presentinvention.

FIG. 5(A) and FIG. 5(B) are plan views which show the shapes of deformedillumination according to the present invention.

FIG. 6 is a plan view which shows an example of a photomask for isolatedholes, in which the auxiliary holes of the past are used.

FIG. 7 is a plan view which shows an example of a photomask for isolatedholes, in which the auxiliary shifters of the past are used.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described in detailbelow, with reference being made to relevant accompanying drawings.

Specifically, FIG. 1(B) and FIG. 1(C) are plan views which show examplesof photomasks 20 and 22 according to the present invention, in which inorder to form a plurality of pattern parts of at least part of thesemiconductor element pattern 25 shown in FIG. 1(A), for example, thebit line contact holes 2 or capacitive contact holes 3, at locationswhich are in opposition to locations at which parts 2 or 3 are to beformed, a plurality of main holes 4 which cause the passage of aprescribed light beam irradiated to each one of portions on which thepatterns 2 or 3 of a plurality of the pattern, is formed, respectively,are provided.

On these photomasks 20 and 22, there are further disposed between themain holes 4 minute auxiliary holes 5 that pass a light beam of a degreethat is not transferred at the time of exposure.

It is desirable that the semiconductor device which uses the photomasks20 and 22 according to the present invention be a memory device, and atleast part of the pattern of this semiconductor device can be eithercontact holes 2 or capacitive contact holes 3, the present inventionpresenting no restriction in this regard.

That is, it is desirable that the main hole 4 and auxiliary hole 5 inthe present invention be arranged so as to form a periodic hole groupwithin the photomasks 20 and 22.

By adopting the above-noted configuration, it is possible, as will bedescribed below, to establish a deep focal depth of field whenperforming exposure.

While the groups of auxiliary holes 5 provided in the photomasks 20 and22 in the present invention are provided at prescribed positions withrespect to each of the groups of main holes 4, this not necessarilybeing restricted to the center part thereof, it is desirable that theybe provided at either the intersection points 7 of virtual matrix lines6 that run between prescribed positions in two directions, or atintersection points 7 of virtual lines 6' that would be formed bydividing the above-noted virtual matrix lines 6 into equal divisions.

The pattern structure that is shown in FIG. 1(A), which is the pattern25 of a semiconductor device that is to be used in photomasks 20 and 22of the present invention, has the same pattern configuration as thepattern structure shown as the prior art in FIG. 3(A) and, in the samemanner, the structure of the photomask 20 for the bit line contactholes, which is shown in FIG. 1(B) corresponds to the photomask 30 forthe bit line contact holes in the prior art as shown in FIG. 3(B), andthe structure of the photomask 22 for the capacitive contact holes shownaccording to the present invention as shown in FIG. 1(C) corresponds tothat of the photomask 32 for bit line contact holes in the prior art, asshown in FIG. 3(C).

That is, the photomask 20 according to the present invention consists ofthe photomask 30 of the prior art, as shown in FIG. 3(B), with theaddition of the auxiliary holes 5, the action of which is tointentionally cause a diffraction of the light beam used for exposure,thereby increasing the focal depth of field.

Specifically, in a first example of the present invention, there is anactivated region 1 having a convex memory cell pattern as shown in FIG.1(A) (the same pattern is formed as shown in FIG. 3(A)) that is formedperiodically and in a concentrated pattern.

When this is formed, the bit line contact holes 2 and capacitive contactholes 3 are disposed so as to be positioned at the points ofintersection 7 of the matrix lines 6 that connect the contact holes.

In the bit line contact hole mask 20 that is shown in FIG. 1(B), theauxiliary holes 5 that are smaller than the resolving limit are disposedat intersection points other than the locations of hole patterns 4.

By doing this, there are holes at all of the matrix points, therebyresulting in a periodic hole pattern.

In the capacitive contact hole mask 22 shown in FIG. 1(C) as well,auxiliary holes 5 smaller than the resolving limit are disposed atmatrix points other than the locations of the hole patterns 4, theresult being that holes are located at all of the matrix points, therebyforming a period hole pattern.

In the present invention, when using the photomask 20 or 22 to expose alight beam to a substrate to form a prescribed semiconductor circuit,with respect to a periodic hole pattern such as shown in FIG. 1(B) orFIG. 1(C), it is particularly effective to use deformed illuminationsuch as shown in FIG. 5(A) or FIG. 5(B), in which the peripheral area isbright.

FIG. 4 shows a simulation comparison of the focal depth of field of atransferred hole diameter obtained using the photomask 22 of the presentinvention that is shown in FIG. 1(B) and the photomask 30 of the priorart that is shown in FIG. 3(B).

Specifically, with a hole pattern 4, the size of which is 0.2 μm and anauxiliary hole 5, the size of which being 0.15 μm, using a ring-shapedlight source such as shown in FIG. 5(A), the light source outer diameterof the light source was σ=0.9 and the inner diameter was σ=0.6.

The numerical aperture of the projection system was 0.5. In the case ofusing a prior art photomask 30 for which the focal depth of field waswithin 0.2±0.02 μm, the transferred hole diameter was 0.4 μm, whereaswith the photomask 20 of the present invention, the transferred holediameter increased to 0.8 μm.

In another form, the photomask of the present invention is a photomaskas shown in FIG. 2(B) and FIG. 2(C), which has a contact hole patternfor a memory device. In the former case, this is the photomask 40 forbit line contact holes, and in the latter case this is the photomask 42for capacitive contact holes, these being the same as the first exampleof the present invention.

Additionally, in the present invention matrix lines 6 are drawn so as tojoin the above-noted contact hole patterns, thereby establishing thematrix points 7 at the locations at which these matrix lines intersecteach other, minute auxiliary holes 5 that are not transferred whenexposure is done being disposed at part or all of these matrix points atwhich a contact hole pattern does not exist, a phase shifter 9 beingplaceed at every other contact hole pattern 4 and auxiliary hole 5 atthe matrix points 7, this phase shifter 9 shifting the phase of lightwhich passes through the contact hole pattern and auxiliary hole 5 by180 degrees.

Specifically, in the present invention, in addition to the main holes 4and auxiliary holes 5 being disposed in a matrix arrangement, an opticalphase shifter means 9 is provided at a hole of one of the main holes 4and the auxiliary holes 5, which being mutually and adjacently arrangedto each other, thereby form a hole with shifter 8 thereat.

The optical phase shifter means 9 of the present invention is notlimited to any particular configuration and can be, for example, aplate-shaped element made chiefly of glass which enables the achievementof a prescribed phase shift in a prescribed location of the holes 4 or 5of the photomask 40 and 42, and can also be configured so as to achievea prescribed phase shift by removing a prescribed amount of thicknessfrom a glass substrate that has the holes of the photomask.

That is, in the second example of the present invention, as shown inFIG. 2(B), in the bit line contact hole mask 40, auxiliary holes 5 aredisposed at matrix point locations 7 at which there is no hole pattern4, these auxiliary holes being smaller than the resolving limit.

By doing this, there are holes 4 and 5 at every matrix point 7, therebyresulting in a periodic hole pattern, with a phase shifter means 9 beingdisposed every other hole pattern 4 or auxiliary hole 5.

In the capacitive hole pattern mask 42 shown in FIG. 2(C) as well,auxiliary holes 5 are disposed at matrix point locations 7 at whichthere is no hole pattern 4, thereby resulting in a periodic hole patternin which there are holes at every matrix point 7.

In the above-noted case as well, there is a phase shifter means 9disposed at every other hole pattern 4 or auxiliary hole 5.

A photomask 40 or 42, as shown in FIG. 2(B) or FIG. 2(C), respectively,is a phase-shift mask that has a periodicity, and it is highly effectiveto use a conventional light source, as opposed to a deformed lightsource, with this type of mask, using high-interference illuminationwith a small σ (that is, a greatly stopped-down light beam in which theratio NA'/NA of the optics numerical aperture NA to the illuminationnumerical aperture NA is small).

Stated in other terms, in the second example of the present invention,it is desirable when using the above-noted photomask 40 or 42, toperform exposure with illumination from an illumination source having anoptical system with a small numerical aperture.

By using the photomask and the exposure method of the present invention,the focal depth of field when transferring contact holes for a memorydevice is extended. Additionally, by broadening the focus margin, it ispossible to effectively form patterns with smaller features, therebymaking possible the exposure of semiconductor integrated circuits havinga higher degree of integration.

What is claimed is:
 1. A photomask for the formation of at least part ofa plurality of pattern parts of a pattern of a semiconductor device,said photomask comprising a plurality of main holes each of which passesa prescribed light beam that is irradiated onto the respective positionsof a substrate, that make up at least a plurality of pattern parts, atlocations that are opposite said respective pattern parts for saidsemiconductor device on said substrate, said photomask furthercomprising auxiliary holes having relatively smaller size with respectto those of said respective main holes, each of which passes a lightbeam of a degree that is not transferred on to a surface of saidsubstrate at the time of exposure, said auxiliary holes being disposedbetween said main holes and wherein said main holes and said auxillaryholes are arranged within said photomask in a periodic arrangement ofholes, and further wherein said auxiliary holes are provided at eitherthe intersection points of virtual matrix lines that run betweenprescribed positions in two directions or at intersection points ofvirtual lines that would be formed by dividing the virtual matrix linesinto equal divisions.
 2. A photomask according to claim 1, wherein saidsemiconductor device is a memory device.
 3. A photomask according toclaim 1, wherein said at least part of a plurality of pattern parts of asemiconductor device are contact holes.
 4. A photomask according toclaim 1, wherein said main holes and said auxiliary holes are arrangedwithin said photomask in a periodic arrangement of holes.
 5. A photomaskaccording to claim 1, said photomask further comprising an opticalshifter means which is provided at at least one hole part of saidplurality of main holes and plurality of auxiliary holes.
 6. A photomaskaccording to claim 5, said optical shifter means is provided on at leastone hole of a pair of said main holes and said auxiliary holes both ofwhich being adjacently arranged to each other.
 7. A photomask accordingto claim 1, wherein said auxiliary holes are provided at theintersection points of virtual matrix lines that run between prescribedpositions in two directions.
 8. A photomask according to claim 1,wherein said auxiliary holes are provided at intersection points ofvirtual lines formed by dividing virtual matrix lines that run betweenprescribed positions in two directions into equal divisions.
 9. Anexposure method whereby a light beam emitted from a light beam source isirradiated on a surface of a substrate through a hole provided on aphotomask and having a prescribed pattern to form a part of prescribedpatterns on said substrate, wherein said method is carried out byutilizing said photomask for the formation of at least part of aplurality of pattern parts of a pattern of a semiconductor device, saidphotomask comprising a plurality of main holes each of which passes aprescribed light beam that is irradiated onto the respective positionsof a substrate, that make up at least a plurality of pattern parts, atlocations that are opposite said respective pattern parts for saidsemiconductor device on said substrate, said photomask furthercomprising auxiliary holes having relatively smaller size with respectto those of said respective main holes, each of which passes a lightbeam of a degree that is not transferred onto a surface of saidsubstrate at the time of exposure, said auxiliary holes being disposedbetween said main holes, wherein a light beam emitted from a deformedillumination source, which is bright around the periphery thereof, isirradiated onto said substrate through either of said main holes or saidauxiliary holes of said photomask, said main holes and said auxiliaryholes being arranged within said photomask in a periodic arrangement ofholes, and further wherein said auxiliary holes are provided at eitherthe intersection points of virtual matrix lines that run betweenprescribed positions in two directions or at intersection points ofvirtual lines that would be formed by dividing the virtual matrix linesinto equal divisions.
 10. An exposure method according to claim 9,wherein illumination having an optics system that has a small numericalaperture is used.
 11. An exposure method according to claim 9, whereinsome of said holes of either one of said main holes or said auxiliaryholes of said photomask are provided with an optical shifter means. 12.An exposure method according to claim 9, wherein said deformedillumination source is annular with an outermost area, a middle area anda inner area, wherein the outermost layer is dark, the middle area is asource of illumination and the inner area is dark.
 13. An exposuremethod according to claim 9, wherein the illumination source is darkenedexcept for four illumination points dispersed in a circular darkenedillumination source.
 14. An exposure method according to claim 9,wherein at least one auxiliary hole causes diffraction of the light beamused for exposure, thereby increasing the focal depth of the field. 15.An exposure method according to claim 9, wherein there are a pluralityof auxiliary holes which are smaller than the resolving limit and aredisposed at intersection points other than the locations of main holepatterns.
 16. An exposure method according to claim 9, wherein a phaseshifter is placed at main holes and auxiliary holes at matrix pointsbetween a contact hole and an auxiliary hole.